Triglycerides from plants and animals, such as vegetable oil and animal fats, consist of long chains of acids which esterify glycerol. These triglycerides have melting points and viscosities that are too great to use in either diesel fuel or jet fuel in conventional engines.
The physical properties of various triglycerides and other oxygenates are summarized in the below table.
BoilingViscosity,CASPoint,MeltingDensity,cStNameFormulaMWID° C.Point, ° C.g/cm3at 40° C.MethanolCH3OH3267-56-164−97.80.79140.58GlycereolC3H8O39256-81-5290(d)18.61.2613310.Acetic AcidC2H4O26064-19-711816.61.04911.16Propanoic AcidC3H6O27479-09-4141−20.80.9920.85Butanoic AcidC4H8O288107-92-6163−6.81.39910.82Methyl EthylanoateC3H6O27479-20-957−980.8740Methyl PropanoateC4H8O288554-12-178.7−87.50.9151Methyl ButanoateC5H10O2102623-42-7102.6<−950.8984Glycerol-triethylanoateC9H14O6218102-76-1259+3.21.15627.02tripropanoateC12H20O6260139-45-7282<−501.100tributylanoateC15H26O630260-01-5308−751.03505.30tripentylanoateC18H32O6344620-68-81.023trihexylanoateC21H38O6386621-70-5386−600.87528.71triheptylanoateC24H44O6428620-67-74700.966trioctylanoateC27H50O6470538-23-8+8.30.9511.57tridecylanoateC33H62O6555621-71-6+320.9220.1tridodecylanoateC39H74O6635538-24-9+460.923.7Glycerol-monoethylanoateC5H10O413426446-35-5−991.2060diethylanoateC7H12O5176105-70-4260401.184dibutanoateC11H20O523232648-01-42731.066As the data in the above table illustrates, the chain length of the triglycerides influences the physical properties of the triglycerides.
For comparison, while the specification properties for diesel fuel and jet fuel can vary between countries and with the season, the typical properties of diesel fuel and jet fuel are summarized in the below table.
TBPBoilingFreezeViscosity,Range,CloudPoint,Density,cStName° C.Point, ° C.° C.g/cm3at 40° C.Diesel145-370−10 to −250.82 to 0.861.9 to 4.1FuelBiodiesel165-180 +2 to +140.86 to 0.903.5 to 5.0Jet Fuel120-290<−400.775 to 0.8401.5 to 3.5Fischer124-374−180.7681.981TropschDiesel
Consequently, in current commercial practice, the long-chain triglycerides are converted to lower viscosity methyl esters by transesterification, typically with a mixture of methanol and a base such as sodium hydroxide. This methyl ester product is commonly called a biodiesel. In general, neat biodiesel (methyl esters) have unacceptably high cloud points and viscosities that are slightly higher than that commonly used in conventional diesel engines. Thus, biodiesel is typically used as a blend with lower cloud point petroleum diesel.
In comparison to the long-chain triglycerides from plants and animals, short-chain triglycerides and cold-climate mixed-chain triglycerides can be used as fuels or fuel blending components. Most short-chain triglycerides have viscosities closer to the values of diesel fuel, can be distilled, and have melting points compatible with diesel fuel.
Another problem with the current approach of making methyl esters, is the by-product glycerol. The transesterification process creates glycerol which can be contaminated with the base used in the preparation of the methyl ester. Glycerol is a highly viscous liquid due to its three hydroxyl groups which interact between molecules via hydrogen bonding. Glycerol is too viscous to be used as diesel fuel. In addition, glycerol has a high melting point and decomposes during distillation. Glycerol has a limited market in some products like explosives (nitroglycerine), cosmetics, and lubricants. The volume of glycerol that will be produced as biofuels grow in production will be in great excess of these limited markets. The yield of glycerol by-product is about ten weight percent of the biodiesel product. Research programs have been launched to find uses for the glycerol by-product. It is desirable to find a way to use the glycerol by-product as a fuel or as a blending component in a fuel.
When glycerol is only partially esterified to form mono- and di-glycerides, the product is still too viscous due to the hydrogen bonding of the remaining hydroxyl groups. While measured viscosities at 40° C. for these compounds have not been reported, they are known to be viscous liquids. The mono- and di-esters of glycerol are known to form emulsions, a property which is generally not desirable in diesel fuel and jet fuels as this makes the separation of water from the fuels difficult.
Another way to convert biomass into a biofuel is to gasify the biomass to make synthesis gas (commonly referred to as “syngas”). The syngas can then be reacted over a Fischer-Tropsch process to make a mixture of compounds that can be converted into fuels by a combination of processes including, but not limited to, the following: hydrocracking, hydroisomerization, polymerization, and combinations thereof. Because of their high content of paraffins, fuels derived from a Fischer-Tropsch process are known to have problems with poor lubricity, low density, and low viscosity.
In addition, the Fischer-Tropsch process makes an equivalent mass of water by-product for the mass of the hydrocarbon product. The water by-product is often contaminated with oxygenates such as alcohols and acids. It is known that the alcohols can be separated from the acid and water. The acid water mixture is typically purified by biological oxidation wherein the acids are consumed by microorganisms. This represents both a loss in product and an inefficiency in the process.
For these reasons, a way to convert glycerol into a useful fuel product; a way to improve the yield of fuel products from biomass synthesis; a way to reduce the loss in products from a Fischer-Tropsch process; and a way to improve the density, viscosity, and lubricity of Fischer-Tropsch fuels is needed.